This invention relates to an optical access system, and more particularly, to an optical access system in which an optical network unit shifts to a sleep state.
In recent years, optical networks are being introduced in order to deal with an increase in the speed and bandwidth of communication networks. An optical network is a system in which one optical line terminal (OLT) and one optical network unit (ONU) communicate with each other via an optical fiber.
Passive optical networks (PONs) are a type of optical network. A passive optical network (PON) system is an optical network system in which one OLT and a plurality of ONUs form a star network, with an optical splitter interposed between the OLT and the ONUs. Typical standards for PONs include Ethernet PON (EPON), which has been standardized by IEEE 802.3.
Uplink frames which are transmitted from the ONUs to the OLT and downlink frames which are transmitted from the OLT to the ONUs are each multiplexed by wave division multiplexing (WDM). A downlink frame is received by every ONU coupled by an optical fiber. Each ONU refers to destination information contained in the preamble part of the downlink frame, and discards the downlink frame if the frame is not destined to itself. An uplink frame in a PON, on the other hand, is multiplexed by time division multiple access (TDMA) to be used for communication.
The communication speed of PONs has been 64 kbit/second at first. In place of systems that handle this low-speed signal, broadband PON (BPON) systems in which ATM cells with a fixed length are transmitted/received at approximately 600 Mbit/second at maximum, EPON systems in which variable-length packets of Ethernet are transmitted/received at approximately 1 Gbit/second at maximum, or even faster gigabit-capable PON (GPON) systems which handle signals of about 2.4 Gbit/second are now being introduced. A further faster PON capable of using signals of 10 Gbit/second to 40 Gbit/second is demanded to be realized in the future.
With the communication speed improved, the power consumption of a relay device along a transmission path is on the rise. A large number of ONUs, which are installed in the homes of and on the premises of subscribers, are set up on a network. On the other hand, an individual ONU needs a band allocated for its use for a shorter period of time than the OLT and a host switch group do. The ONU is thus left idle wasting power during a period in which the ONU is not communicating.
JP 2008-113193 A, for example, discloses a method of cutting power consumption in which, in order to reduce power consumption during a non-communication period of an ONU, function blocks within the ONU are set to a low power consumption mode if terminal equipment (TE) is not coupled to the ONU via a LAN cable.
JP 2009-260970 A, for example, discloses a method involving setting an ONU to a sleep state through a procedure in which a sleep time request of the ONU is granted by the OLT.
As described above, the increased demand for high-speed, large-capacity communication of recent years has put the power consumption of a relay device along a transmission path such as an ONU on the rise, and a way to make the relay device operate on low power is being looked for. The technology disclosed in JP 2008-113193 A is a technology of monitoring only for whether or not the TE is coupled to a LAN port, and shifting to and returning from the low power consumption mode in accordance with the monitoring result. The technology disclosed in JP 2008-113193 A therefore cannot reduce the power consumption of the ONU once the TE is coupled to the ONU, and lets the ONU consume as much power as in normal operation when the ONU is not actually communicating.
The technology disclosed in JP 2009-260970 A has an additional procedure in which an ONU shifts to a sleep state during a non-communication period, and is thus capable of reducing the power consumption of an ONU with the TE coupled thereto during a non-communication period. However, with the technology disclosed in JP 2009-260970 A, the ONU remains in a sleep state only for a given length of sleep time and, when a short sleep time is set, for example, the ONU frequently repeats the shift to and recovery from a sleep state, which limits the effect of the technology in reducing the power consumption of the ONU.